Generally, electrostatographic imaging is performed in cycles by forming a latent image of an original document onto a substantially uniformly charged photoreceptive member. The photoreceptive member has a photoconductive layer. Ordinarily, exposing the charged photoreceptive member with the image discharges areas of the photoconductive layer corresponding to non-image areas of the original document, while maintaining the charge in the image areas or vice versa. In discharge area development, the reverse is true where the image areas are the discharged areas and the non-image areas are the charged areas. Thus in either case, a latent electrostatic image of the original document is created on the photoconductive layer of the photoreceptive member.
Charged developing material is subsequently deposited on the photoreceptive member to develop the latent electrostatic image areas. The developing material may be a liquid material or a powder material. The charged developing material is attracted to charged or discharged latent electrostatic image areas on the photoconductive layer. This attraction develops the latent electrostatic image into a visible toner image. The visible toner image is then transferred from the photoreceptive member, either directly or after an intermediate transfer step, to a copy sheet or other support substrate as an unfused toner image which is then heated and permanently affixed to the copy sheet, resulting in a reproduction or copy of the original document. In a final step, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material in order to prepare it for successive imaging cycles.
In full process color electrostatographic printing, rather than forming a single latent image on the photoconductive surface, separate latent images, corresponding to different color separations, must be created. Each single color latent electrostatic image is developed with a corresponding colored toner. This process is repeated for a plurality of colors. By any one of several processes, each single-color toner image is eventually superimposed over the others and then results in a single full process color toner image on the copy sheet. Thereafter, the full process color toner image is also heated and then permanently fixed to a copy sheet, creating a full-color copy.
In a conventional tandem color printing process, four imaging systems are typically used. Photoconductive drum imaging systems are typically employed in tandem color printing due to the compactness of the drums. Although drums are used in the preferred embodiments, a tandem system can alternatively use four photoconductive imaging belts instead of the drums. Each imaging drum or belt system charges the photoconductive surface thereof, forms a latent image thereon, develops it as a toned image and then transfers the toned image to an intermediate belt or to a print medium. In this way, yellow, magenta, cyan, and black single-color toner images are separately formed and transferred. When superimposed, these four toned images can then be fused, and are capable of resulting in a wide variety of colors.
In image-on-image color printing, an endless photoreceptor belt, a controller and a series of imaging subassemblies are employed that each include a charging unit, a color separation latent image exposure ROS unit or LED print bar, and a corresponding color toner development unit. As the endless photoreceptor belt moves in an indicated direction, an image frame thereon is charged, exposed and developed, in succession, by each imaging subassembly, with each imaging subassembly thus forming a color separation image corresponding to color separation image input video data from the controller. After the first imaging subassembly forms its color separation toner image, that color separation toner image is then recharged and re-exposed to form a different color separation latent image, and then correspondingly developed by the next imaging subassembly. After the final color separation image is thus formed, the fully developed full process color image is then ready to be transferred from the image frame at transfer station to a print media.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present disclosure. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, are intended to provide a better understanding and appreciation of the present disclosure.
U.S. Pat. No. 5,347,353 issued Sep. 13, 1994 to Fletcher and entitled “Tandem high productivity color architecture using a photoconductive intermediate belt” discloses a system in which tandem, high productivity color images are formed by using a photoconductive belt as an imaging surface and as a transferring device. A full process colored image is produced comprising a plurality of color layers. The apparatus includes a charging device, an image forming device, and a developing device located along a photoconductive belt to form a toned image layer on the belt. Additional color layers may be provided by either photoreceptive imaging drums or additional photoconductive belts.
U.S. Pat. No. 5,837,408 issued Nov. 17, 1998 to Parker et al. and entitled “Xerocolography tandem architectures for high speed color printing” discloses a full process color imaging system that uses two xerocolography engines in tandem. Each of the two xerocolography engines is capable of creating three perfectly registered latent images with subsequent development thereof in a spot next to spot manner. Each engine is provided with three developer housing structures containing five different color toners including the three subtractive primary colors of yellow, cyan and magenta. Two of the primary colors plus black are used with one of the engines. The third primary color is used with the second tandem engine which also uses one of the primary colors used with the first engine as well as a fifth color which may be a logo or a gamut extending color. The full process color imaging capability provided is effected without any constraints regarding the capability of the laser imaging device to image through previously developed components of a composite image. Also, the development and cleaning field impracticalities imposed by quad and higher level imaging of the prior art are avoided. Moreover, the number of required image registrations compared to conventional tandem color imaging is minimal. Therefore, only one registration is required compared to three or four by conventional tandem engine imaging systems.
U.S. Pat. No. 5,613,176 issued Mar. 18, 1997 to Grace and entitled “Image on image process color with two black development steps” discloses a printing system using a recharge, expose and development image on image process color system in which there is an optional extra black development step. The printing system may be a system where all of the colors are developed in a single pass, or a multi-pass, system where each color is developed in a separate pass. The additional black development step results in optimal color quality with black toner being developed in a first and/or last sequence. Having more than one black development station allows low gloss and high gloss black toner to be applied to the same image, enabling the very desirable combination of low gloss text and high gloss pictorials on the same page.
U.S. Pat. No. 5,296,904 issued Mar. 22, 1994 to Jackson and entitled “Three-roll fuser with center pressure roll for black and color application” discloses a three roll fuser system for a xerographic machine includes a reversibly drivable central pressure roll, a first fuser roll located adjacent the central pressure roll forming a first fuser nip with the central roll, and a second fuser roll located adjacent the central pressure roll on a substantially opposite side of the central pressure roll as the first fuser roll forming a second fuser nip with the central roll. Copy sheets having an unfused image on a side thereof are transported from an inlet through one of the first and second nips to fuse the image on the copy sheet and then transported to an outlet. The three roll fuser system is capable of selectively fusing either side of a copy sheet without requiring extra sheet inverting devices. In a preferred embodiment, the fuser rolls have differing physical properties and can be operated under different operating conditions such as fuser temperature and speed.
In conventional color printing systems with black only image capability, it is well known that the run cost of the color xerographic print engine is much higher than that of a stand alone monochrome black print engine, even when only black images—are being produced. This higher run cost issue has been identified as one of the barriers to greater and faster color printing systems adoption in the office and in lower-volume production markets where providing both a color and monochrome black engine may not be justifiable. This higher run cost issue is also an annoyance to high-volume production customers because incorporating pages from a stand alone low cost monochrome black engine into a mixed job may be even more expensive than printing black pages at the higher run cost on their color print engine.
Conventional printing systems such as those described above can nowadays be found in the office environment as well as in small or entry production environments. The trend by manufacturers however is towards slower color image producing versions that also offer a limited form of “black images” only from the color version. The black image production is limited because color version printers (including the current conventional ones that also offer black images) tend to run at higher run costs per print even when running black images only or in a black mode. The undesirable result is additional wear to the color components as well as higher run costs for each print, color or black.
There is therefore a current need for a printing system that can produce color images as well as black images without the current disadvantages of slower speeds and higher costs for the black images.